1. Field of the Invention
The present invention relates generally to a composite masking system and method for improving the invisibility of high-definition video watermarking and, more particularly, to technology that can improve the perceptual quality of a video into which a watermark has been embedded using a composite mask, including a Noise Visibility Function (NVF) mask, an adaptive dithering mask and a contour mask, which takes into consideration a Human Visual System (HVS).
2. Description of the Related Art
With the growth of the infrastructure and industry related to digital video content, illegitimate copies and distributions of video content have increased because they can be easily processed, delivered and stored.
Since such illegitimate acts result in great financial harm to content providers and the related market, video watermarking has become important as a digital right management system.
As an alternative, there is a method of tracking an illegitimately reproduced copy back to the receiver from which it originated, using a technique for imperceptibly inserting a signal, including the copyright information of the invisible video watermark, into the original video content.
Recently, the popularization of high-resolution video content requires such a video watermarking system to meet several requirements: invisibility, robustness, and real-time processing. First, a watermark embedded in video content should be imperceptible by a human observer. The higher the quality of video content, the more important is invisibility. Second, high-resolution video content is generally manipulated several different times so that it can be adapted to a variety of display devices, such as an LCD TV, a portable multimedia player, and a high-performance mobile phone. In a practical situation, the manipulations mostly include downscaling, trans-coding into various formats, and frame rate conversion. Detection of the embedded watermark should be reliable in spite of major manipulations. Finally, the computational expense of video watermarking should be low. A real-time video watermarking system is especially required for the protection of high-quality video on-demand (VOD) services.
Each element of a watermark pattern that is to be embedded should be repeatedly expanded so that it can be reliably detected in a watermarked video even when scaled to a small size. However, reinforcing the low frequency of the extended pattern causes block artifacts, resulting in deterioration in the invisibility of the embedded watermark pattern.
FIG. 1(a) is an original image, FIG. 1(b) is an image in which a pattern which has not been extended has been embedded, and FIG. 1(c) is an image in which an 8 times extended pattern has been embedded. As shown in FIGS. 1(a) to 1(c), the block artifacts are clearly shown in the case of being extended 8 times. Although the peak signal-to-noise ratios (PSNR) of FIG. 1(b) and FIG. 1(c) are equally 38 dB, it is obvious that the watermark invisibility of FIG. 1(c) is lower.
In order to solve the block artifact problem, a method using dithering masking was proposed. However, in this method, all watermark patterns of the embedded regions are dithered by masking by an equal amount without considering the property of the cover work, and therefore there is a limit as to how much the watermark embedding strength can be controlled and consequently the performance of the imperceptibility decreases.
Meanwhile, hitherto a noise visibility function (NVF) masking based on an HVS has been utilized. According to the NVF, the watermark can be embedded more strongly in contour or texture regions than in flat regions.
However, contour or edge regions are more sensitive to noise addition than texture regions but less so than flat regions according to the HVS, and thus the traditional masks such as the NVF mask have the problem of not distinguishing between the contour region and the high texture region. If the traditional mask is used directly, the contour region can be easily corrupted and it results in severe distortions to the video.